1. Field of the Invention
The invention relates to an inverter comprising a direct-voltage source and a chopper section which has switching means which operate in push-pull and which are switched at the frequency f.sub.O, a parallel tuned circuit being provided between the chopper section and the direct-voltage source.
2. Prior Art
In many fields of electrical engineering, it is necessary to obtain a direct current which is as smooth as possible for feeding a device from an alternating-current system. An example of this is the heating of the electron beam gun of an electronic welding machine. If this cathode is heated with a direct current with too much ripple, the electron beam cannot be accurately controlled.
It is known that the ripple of a direct current obtained from a 50-Hz alternating-current system can be reduced by filtering means, particularly by capacitors. It is also known to reduce the ripple by increasing the frequency. In this arrangement, the 50-Hz alternating-current system frequency is transformed, for example, into the kilohertz range.
As a rule, the frequency increase is effected by simple rectification of the system alternating voltage with subsequent high-frequency chopping of the rectified voltage. The smoothing effort required for obtaining an almost ideal direct voltage is much less if the frequency is increased. However, the power loss occurring at the switching means of the chopper during the switching process is relatively great and this has unfavorable effects particularly when high powers are to be switched.
In the above mentioned cathode heating supply, about 300 watts must be switched at a transformer having a large leakage inductance and the cathode, which is at -60 kV, must be supplied with direct current which is as smooth as possible.
Numerous inverters are already known which convert a direct current into alternating current by means of an electronic chopper (German patent specification No. 2,713,072, German Offenlegungsschrift No. 3,142,304). However, these inverters do not have a parallel tuned circuit between the chopper section and the direct-voltage source.
However, inverters are also known which contain a parallel tuned circuit (German patent specification No. 2,453,924, German Offenlegungsschrift No. 3,142,304, European patent EP-A 005083, Japanese patent journal: Patents Abstracts of Japan, E-204, Oct. 15, 1983, Vol. 7, No. 33, 58-123369 (A)).
However, the resonant frequencies of the respective parallel tuned circuits in these arrangements have a non-optimal relationship to the frequencies with which the switching means of the inverter are switched.
The invention is based on the object of significantly reducing the power loss of the switching means of the chopper section during the switching process in an inverter.
This object is achieved by the fact that the resonant frequency of the parallel tuned circuit is set to 2f.sub.O.
The advantage achieved by means of the invention consists, in particular, in the fact that the switching means, which operate in push-pull, are always switched over when the voltage of oscillation of the parallel tuned circuit is zero. Since the power loss during the switching is defined by the product of current and voltage within this period of time, this greatly reduces the switching power loss.